Note that cluster_awrite() always bawrite()'s or equivalent. The
DragonFly API split the functions out, so cluster_write() can now
almost always bdwrite() for the non-clustered case.

* Solves some serious performance and real-time disk space usage issues
when HAMMER1 was updated to use the cluster calls. The disk space
would be recovered by the daily cleanup but the extra writes could
end up being quite excessive, 25:1 unnecessary writes vs necessary
writes.